Balanced frequency detector



June 9, 1942. H. A. WHEELER BALANCED FREQUENCY DETECTOR Filed March 29, 1941 INVENToR ROLD A. WHEEL R ATTORNEY Patented une 9, 1 942 UNITED STATES PATENT OFFICE 2,285,957 BALANCED FREQUENCYDETECTOR Harold A. Wheeler, Great Neck, N. Y., assigner to Hazeltine Corporation, a corporation of Dela- Application March 29, 1941, Serial No. 385,801

(Cl. Z50-27) 7 Claims.

The present invention relates to an improved frequency-detector system for carrier-signal receivers and, particularly, to such a system of the balanced type. While the invention is of general application, it has particular utility as the frequency-detector system of a frequencymodulated carrier-signal receiver. The invention, therefore, will be described in connection with such a receiver wherein itis effective greatly to reduce the effect of received noise -disturbances.

Frequency-modulation systems are subject to noise disturbances which generally produce incidentally an undesirable amplitude modulation of the carrier signal. While the amplitudelimiting system sometimes used in frequencymodulated receivers may reduce the effect of the noise disturbances, it cannot completely do so at all times and under all conditions of operation, especially when the received carrier signal is relatively weak. A carefully balanced frequency detector is a valuable adjunct to a limiter system in further reducing the effect of noise disturbances and is even more valuable if no limiter is used. A balanced frequency detector exhibits its greatest noise-reducing properties during the intervals when the carrier signal is unmodulated, at which time the noise is most noticeable, and particularly so when the carrier signal is of relatively low intensity. Y

The most common frequency-modulation detector of the prior art includes a slope filter comprising a transformer having a primary winding included in the anode circuit of a preceding repeater vacuum tube and having a center-tapped secondary winding', the center tap being coupled by a radio-frequency condenser to the high potential terminal of the primary winding. The terminals of the secondary winding are connected to a pair of rectifier devices each having an output load impedance and a common junction therefor. Such prior art detectors include an output circuit having the aforesaid load impedances, with suitable by-pass condensers, serially arranged therein, one terminal of the series circuit being conventionally grounded. The low potential terminal of the transformer primary winding is conventionally by-passed to ground through a condenser to provide a lowimpedance path for carrier-signal currents around the anode space current supply source conventionally connected to this transformer terminal. Since the center tap of the secondary winding has an audio-frequency coupling to the common junction of the rectifier load imped- 55 ances, this coupling normally comprising a radiofrequency choke coil, the aforementioned radiofrequency condenser is effectively connected through the by-pass condenser across the load impedance of one of the rectifier devices, thereby to unequalize the impedance components of the rectifier' load impedances. The radio-frequency coupling condenser ordinarily has a value large in comparison to the capacitance of the load impedances and the latter cannot, therefore, be well proportioned to avoid the unequalization thus effected. This results in the production of unequal voltages across the two load impedances at the higher audio frequencies which are predominant in undesired noise disturbances accompanying the carrier signal. The unequal noise voltages thus cannot be balanced out in the` output circuit for this type of prior art frequency detector.

It is an object of the present invention, therefore, to provide an improved frequency-detector system of the balanced type which, while of general applicatiom is especially suitable for use as the frequency-detector system of a frequencymodulated carrier-signal receiver and one which avoids one or more of the above-mentioned disadvantages and limitations of the prior art devices.

It is a further object of the invention to provide an improved frequency-detector system of simple circuit arrangement comprising a minimum number of circuit elements and one which has substantially zero response to spurious amplitude modulation and derives substantially zero output from a carrier signal having no frequency modulation, whereby the effect ofnoise disturbances is 4greatly reduced regardless of the strength of the receivedcarrier signal.

In accordance with the invention, a balanced frequency detector comprises a slope filter including a transformer and means for deriving voltages equal to the sum and difference of voltages developed in the windings of the transformer by a carrier signal applied thereto. The detectorl includes a pair of rectifier devices so coupled to the slope lter as to have applied thereto individual ones of the sum and difference voltages, and a load network for each of the rectifier devices. The load networks have a common junction and the detector has an inherent impedance to ground tending to render the impedance components of said load network nonsymmetrical relative to the common junction and means are included in one of the load networks for compensating the inherent capacitance to render the impedance components symmetrical relative to the junction, whereby undesired transient amplitude disturbances of the applied carrier signal develop equal voltages of opposite polarities across the load networks with respect to the aforesaid common junction at the mean frequency of the applied carrier signal. The detector additionally includes an output circuit coupled to the load networks for deriving a voltage equal to the difference of the voltages developed across the load networks.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accom-' panying drawing, and its scope will be pointed out in the appended claims.

Referring now to the drawing, Fig. 1 is a circuitdiagram, partly schematic, of a complete frequency-modulated carrier-signal receiver of the superheterodyne type embodying the invention; Fig. 2 represents a suitable slope-filter transformer construction for the Fig. 1 circuit; Fig. 3 is a circuit diagram of a portion of a carrier-signal receiver embodying a modified form of the invention; and Fig. 4 is a slope-filter transformer construction suitable for the circuit of Fig. 3.

Referring now more particularly to Fig. 1, there is represented schematically a complete frequency-modulated carrier-signal receiver of the superheterodyne type and of conventional design embodying the present invention in a preferred for-m. In general, the receiver includes a radio-frequency amplifier I0 having its input circuit connected to an antenna system Il, l2 and having its output connected to an oscillatormodulator I3. Connected in cascade with the oscillator-modulator, in the order named, are an intermediate-frequency amplifier I4 of one or more stages, a frequency detector I5 more fully described hereinafter, an audio-frequency amplifier I6 of one or more stages, and a sound reproducer I1. An automatic amplification control or A. V. C. rectifier 9 also is connected to the intermediate-frequency amplifier I4 and the output of the rectifier is applied to the input circuits of one or more of the tubes of the radio-frequency amplifier Ill, the oscillator-modulator I3, and the intermediate-.frequency amplifier I4 in con- Ventional manner.

It will be understood that the various units just described may, with the exception of the frequency detector I5, be of a conventional construction and operation, the details of which are known in the art, rendering further detailed description thereof unnecessary. Considering briefly the operation of the receiver as a whole, and neglecting for the moment the detailed operation of the frequency detector I5 presently to be described, a desired frequency-modulated carrier signal is selected and amplified by the radio-frequency amplifier Ill, converted to a frequency-modulated intermediate-frequency carrier signal in the oscillator-modulator I3, amplified in intermediate-frequency amplifier I4, and detected by the frequency detector I5, thereby to derive the audio-frequency modulation components; The audio-frequency components are, in turn, amplified in the audio-frequency amplifier I6 and are reproduced by the sound reproducer I1 in a conventional manner. The intermediatefrequency carrier signals also are applied to the automatic amplification control or A. V. C. rectilier 9 to derive an automatic control or A. V. C.

bias. This bias is effective to control the amplification of one or more of the units I9, I3, and I4 to maintain the signal input to the detector I5 within a relatively narrow range for a wide range of received signal intensities.

Referring now more particularly to the portion of the system embodying the present invention, the frequency detector I5 includes a slope filter I8 comprising a transformer having a first winding I9 included in the output circuit of the intermediate-frequency amplifier I4, a second winding 2|) coupled to the first winding I9, and a center-tapped third winding 2I coupled to Winding 29 but substantially uncoupled to winding I9 so that the second winding 20 provides substantially the entire energy transfer from the first winding I9 to the third winding 2|. The second and third windings 29, 2| of the slope-filter transformer are tuned by the respective condensers 22, 23 to the mean frequency of the intermediate-frequency modulated-carrier signal of the receiver. The detector I5 includes means for deriving voltages equal to the sum and difference of the voltages developed in the windings 29, 2i of the slope-filter transformer comprising a connection between the center tap of the transformer winding 2I and a terminal of the transformer winding 29, a pair of rectifier devices 26, 21, which are individually connected with like polarity between the terminals of winding 2| and the other terminal of winding 20 so as Y The frequency detector I5 is provided with an output circuit comprising an output terminal 34 and a grounded output terminal 35 with theload networks 28, 29 included in series therebetween to derive a voltage equal to the difference of the voltages developed across the load networks. As thus arranged, the load networks 28, 29 have a common junction 3S which is connected to one terminal of the transformer winding 20 of the slope filter I8. The circuit to the common junction 36 has an inherent capacitance to ground, represented by the broken-line condenser 3l., which is effectively in shunt to the condenser 33 of the load network 29 and tends undesirably to render unsymmetrical the impedance components of the load circuit of the frequency detector.

Considering now the operation of the frequency-detector system just described, the detector is initially adjusted for balanced operation by adjusting the condenser 33 to such value that the sum of the capacitance of the condenser 33 and the inherent capacitance of the condenser 31 is equal to the capacitance of the condenser 3|. When this is done, the sums of all like physical and inherent impedance components ofthe detector are symmetrical with respect to the common junction 36. The adjustable condenser 33 thus comprises means included in one of the load networks for compensating the inherent capacitance at the common junction 36 to equalize the total capacitance across the load networks 28, 29, or to render the impedance components symmetrical relative to the junction 36. The slope nlter I8 derives from the intermediate-frequency carrier signal applied thereto two voltages varying in magnitude in opposite senses with the frequency of the carrier signal. These voltages, the sum and difference voltages of the transformer windings of the slope filter I8, are applied to in divldual ones of the rectifier devices 26, 2l to develop across the load networks 28, 29 unidirectional voltages of opposite polarlties with respect to the common junction 36 and of magnitude varying in opposite senses with the frequency of the carrier signal. The unidirectional voltages 4thus developed are combined in the output circuit of the frequency detector I5 to derive a modulation voltage equal to the difference of the 'voltages developed across the load networks 28,

' 28. The modulation voltage is applied to the in- Since all impedance components of the frequency detector are symmetrical with respect to the common junction 36, as pointed out above, undesired transient amplitude disturbances of the applied intermediate-frequency carrier signal develop equal transient unidirectional voltages of opposite polarities across the load networks 28, 28 at the mean frequency of the applied carrier signal. These transient voltages balance out in the output circuit of the detector I5 and consequently are not reproduced by the sound reproducer Il.

Fig. 2 illustrates a suitable construction for the transformer of the slope filter I8 in the Fig. l embodiment of the invention. The first winding I8 is a multilayer winding relatively closely coupled to the second Winding 2D, which is of the single-layer winding type. The third winding 2| is also a multilayer winding and is relatively loosely coupled to the winding 28 in order to reduce the coupling between windings I9 and 2i to a minimum. The three windings are supported on a common tubular insulating member 38 and the transformer is enclosed within a shield 40.

As illustrative of a specific embodiment of the invention, the following circuit constants are given for an embodiment of the invention of the type shown in Fig. 1:

. Millihenries Inductance of transformer winding I9 48 Inductance of transformer winding 37 Inductance of transformer winding 2L- 43 External diameter of support 38 lnch 0. 75 Spacing between windings I3 and zu ;inch o, 0625 Spacing between transformer windings 20 and 2I lnch 0. 343 vLength of winding 20 do 0.375 Rectifier devices 26, 2'I Type 7A6 Resistors and 32 ohms-- 220, 000 Condenser 3| micromicrofarads 500 Condenser 33 do 100-500 Fig. 3 is a circuit diagram of a portion of a complete signal-carrier receiver and represents a modified form of the detector system which is essentially similar to that of the Fig. 1 arrangeseparate transformers il and 42, the transformer modification of the invention differs also from that of Fig. 1 in that the inherent capacitance 31 of the circuit of the common junction 36 is compensated by an adjustable condenser 38 connected in shunt to the condenser 3I of the load network 28. The corresponding circuit elements of the load networks 28, 29 have equal values of resistance and capacitance, and the condenser 33 is initially adjusted, in balancing the frequency detector I5, to a value of capacitance equal to that of the inherent capacitance 3l, whereby the frequency detector has physical and inherent impedance components symmetrical with respect to the common junction 36. The operation of this modification of the invention is essentially simi'- lar to that of Fig. l, the particular form of slopen filter transformer arrangement ensuring mini` mum coupling between the transformer windings I9 and 2| and will, therefore. not be repeated.

Fig. 4 illustrates a suitable construction for the transformer 42 of the slope filter I8 in the Fig. 3 modification of the invention. The `ransformer winding 2 I, which is shown in schematic form, is

,'- symmetrically disposed 0n the center of an insulating winding form 38 and the two halves of the winding 2I are bifilar-wound. 'I'he transformer winding 20 is also wound in two sections, one on each side of the Winding 2 I. The two sections of the winding 2li are wound in opposite directions and are connected in parallel with aiding mutual inductance, as shown. This transformer construction is of the type disclosedby applicant in the proceedings of the Institute of Radio Engineers, April 1928, pages 476-481.

While there have been 4described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modications as fall within the true spirit and scope of the invention.

What is claimedis:

1. A balanced frequency detector comprising, a slope filter including a transformer and means for deriving voltages equal to the sum and difference of voltages developed in the windings lof said transformer by a. carrier signal applied thereto, a pair of rectifier devices so coupled to said slope filter as to have applied thereto individual ones of said sum and difference voltages, a. load network for each of said devices, said load networks having a. common junction and said detector having an inherent capacitance to ground tending to render the impedance components of said load networks non-symmetrical relative to said junction, means included in one of said load networks for compensating said inherent capacitance to render said impedance components symmetrical relative to said junction whereby undesired transient amplitude disturbances of said applied carrier signal develop equal voltages of opposite polarities across said load networks with respect to said common junction at the mean frequency of said applied carrier signal, andv an output circuit coupled to said load networks for deriving a voltage equal to the difference of the voltages developed across said load networks.

2. A, balanced frequency detector comprising, an input circuit adapted to have a carrier signal applied thereto, a slope filter including a transformer having a rst winding coupled to said input circuit and having second and third coupled windings, one of said second and third windings being coupled to said first winding and said second and third windings being connected to derive voltages equal to the sum and difference of voltages developed therein by said applied carrier signal, a pair of rectifier devices so coupled to said slope filter as to have applied thereto individual ones of said sum and dierence voltages, a load network for each of said devices, said load networks having a common junction and said detector having an inherent capacitance to ground tending to render the impedance components of said load networks non-symmetrical relative to said junction, means included in one of' said load networks for compensating said in- Y herent capacitance to render said impedance components symmetrical relative to said junction whereby undesired transient amplitude disturbances of said applied carrier signal develop equal voltages of opposite polarities across said load networks with respect to said common junction at the mean frequency of said applied carrier signa1,.and an output circuit coupled to said load networks for deriving a voltage equal to the difference of the voltages developed across said load networks;

3.V A balanced frequency detector comprising, an input circuit adapted to have a carrier signal applied thereto, a slope filter including a transformer having a first winding coupled to said input circuit and having second and third windings coupled to derive voltages equal to the sum and difference of voltages developed therein by said carrier signal, said second winding providing substantially the entire energy .transfer from said first winding to said third winding, a pair of rectifier devices so coupled to said slope filter as to have applied thereto individual ones of said sum and difference voltages, a load network for each of said devices, said load networks having a common junction and said detector 4having an inherent capacitance to ground tending to render the impedance components of said load networks non-symmetrical relative to said junction, means included in one of said load networks for coinpensating said inherent capacitance to render said impedance components symmetrical relative to said junction whereby undesired transient amplitude disturbances of' said applied carrier signal develop equal voltages of opposite polarities across said load-networks with respect to said common junction at the mean frequency of said applied carrier signal, and an output circuit coupled to said load networks for deriving a voltage equal to the difference of the voltages developed across said load networks.

4. A balanced frequency detector comprising, a slope lter including a transformer and means for deriving voltages equal to the sum and difference of voltages developed in the windings of said transformer by a carrier signal applied thereto, a pair of rectifier devices so coupled to said slope filter as t have applied thereto individual ones of said sum and difference voltages, a load network for each of said devices, said rectifier devices being connected in said rectifier systems with like polarity with respect to said slope filter, said load networks having a common junction and said detector having an inherent capacitance to ground tending to render the impedance components of said load networks non-symmetrical relative to said junction, means included in one of said load v,networks for compensating said inherent capacitance to render said impedance components symmetrical relative to said junction whereby undesired transient amplitude disturbances of said applied carrier signal develop equal voltages of opposite polarities across said load networks with respect to said common junctionv at the mean frequency of said applied carrier signal, and an output circuit coupled to said load networks for deriving a voltage equal to the difference of the voltages developed across said load networks.

5. A balanced frequency detector comprising, a slope filter including a transformer and means for deriving voltages equal to the sum and difference of voltages developed in the windings of said transformer by a carrier signal applied thereto, a pair of rectifier devices so coupled to said slope lter as to have applied thereto individual ones of said sum and difference voltages, a load network for each of said devices, said load networks having a common junction, and said detector having an inherent capacitance to ground effectively in shunt relation to one of said load networks tending to render the impedance components of said networks non-symmetrical relative to said junction, means included in said one load network to' render the sums of all physical and inherent impedance components of the frequency detector symmetrical with respect to said common junction, whereby undesired transient amplitude disturbances of said applied carrier signal develop equal voltages of opposite polarities across said load networks with respect to v said common junction at the mean frequency of said applied carrier signal, and an output circuit coupled to said load networks for deriving a voltn age equal to the difference of the voltages developed across-said load networks.

6. A balanced frequency detector comprising, a slope' filter including a transformer and means for deriving voltages equal to the sum and difference of voltages developed in the windings of said transformer by a carrier signal applied thereto, a pair of rectifier devices so coupled to said slope filter as to have applied thereto individual ones of said sum and difference voltages, a load network for each of said devices, said load networks having a common junction, an output circuit coupled to said load networks and having a grounded terminal for deriving a voltage equal to the difference of the voltages developed across said load networks, the circuit to said common junction having inherent capacitance to ground which tends undesirably to render unsymmetrical the impedance components' of said load networks with respect to said junction, and means for compensating said inherent capacitance to equalize the total capacitance across said load networks, whereby undesired transient amplitude disturbances of said applied carrier signal develop equal voltages of opposite polarities across said load networks with respect to said common junction at the mean frequency of said applied carrier signal.

7. A balanced frequency detector comprising, a slope filter including a transformer and means for deriving voltages equal to the sum and differ- -ence of voltages developed in the windings of said transformer by a carrier signal applied thereto, a pair of rectifier devices so coupled to said slope filter as to have applied thereto individual ones of said sum and difference voltages, a load network for each of said devices, said load networks having a common junction, an output circuit coupled to said load networks and having a grounded terminal for deriving a developed across said load networks, the circuit to said common junction having inherent capacitance to ground which tends undesirably to render unsymmetricai the impedance components of said load networks with respect to said junction, and an adjustable condenser connected across one oi said load networks for compensating said inherent ,capacitance to equalize the voltage equal to the diilerence oi the voltages total capacitance across said load networks, whereby undesired transient amplitude disturbances of said applied carrier signal develop equal voltages o! opposite polarities across said load networks with respect to said common junction at the mean frequency of said applied carrier signal.

HAROLD A. WHEELER. 

